The present invention generally relates to a semiconductor memory device, and in particular to a semiconductor memory device having a register in addition to memory cells. More particularly, the present invention relates to a random access memory device having a register which can be suitably used as a memory for image processing.
Generally, data write-in and read-out operations with respect to a memory cell of a conventional random access memory device (hereafter simply referred to as a RAM device) are carried out per a predetermined data unit consisting of 1, 4 or 8 bits, (for example). However, such operations cannot sufficiently satisfy various needs of customers. In particular, in the field of image processing, it is required to employ a RAM device which has a very large amount of the storage capacity and can transfer data at a high-speed. Currently, a RAM device capable of satisfying the above requirement has been proposed. For example, 64 kbit and 256 kbit dynamic random access memory devices exclusively designed for the image processing are available.
As the memory capacity per one semiconductor device increases, there exists another requirement for a higher-grade random access memory capable of transferring data at a higher speed and also performing the image processing more simply. From this viewpoint, currently there has been considerable activity in the development of a RAM device which has a register built therein in addition to memory cells to thereby provide special functions. For example, the built-in register may be used as a mask register, which can control, per one bit unit, data to be written into a memory cell. Further, the use of the built-in register makes it possible to update, within one reference cycle, data made up of a predetermined number of bits which exceeds a number of input pins of the RAM device. The RAM device having these special functions can perform the complex image processing at a high-speed. However, it is to be noted that it is impossible to transfer data stored in the built-in register to an external circuit and to directly know the contents of the built-in register. Therefore, validiting the operation of the RAM device becomes very complicated. For example, it is very difficult to find whether an erroneous operation arises from the built-in register or data itself. This disadvantage leads to an increase in a time term taken to develop the RAM device, a more complicated testing procedure thereof, and an increase in cost.